Toasted-Marshmallow Topping

Stir corn syrup, 1 cup water, and sugar in medium saucepan over medium-high heat until sugar dissolves. Attach candy thermometer to side of pan. Boil syrup without stirring until thermometer registers 238 °F, occasionally brushing down sides of pan with wet pastry brush, about 12 minutes. Meanwhile, place egg whites, cream of tartar, and vanilla in bowl of heavy-duty mixer fitted with whisk. When thermometer in syrup registers 230 °F, beat whites at medium-low speed until peaks form. When thermometer registers 238 °F, pour hot syrup down sides of bowl, beating until marshmallow topping is shiny and thick, about 2 minutes. Scrape topping into glass pie dish. DO AHEAD Can be made 1 day ahead. Cover with inverted bowl (plastic wrap will stick). Let stand at room temperature. If topping hardens, microwave in 10-second intervals before browning. Preheat broiler. Broil marshmallow topping until surface is golden brown, about 1 minute. Serve immediately.